A known track on the undercarriage of a track-type vehicle (such as a hydraulic excavator, bulldozer et cetera) comprises endlessly-coupled track links and track shoes mounted on the respective track links. The track is passed around a sprocket and an idler which are supported on the vehicle frame and disposed with a required spacing. Engaged with the sprocket of the drive wheel, each track link is driven. The track is supported by track rollers and carrier rollers disposed between the sprocket and the idler in the vehicle frame. The track-type work vehicle travels by such arrangement.
As shown in FIGS. 9(a) and 9(b), a track link of the track of the above-described type is, in general, a single link (a link assembly 70) formed by interconnecting offset type link pieces 71, 71, which are formed symmetrically relative to the width-wise direction, by a track bushing 72, with a space left therebetween. Such link assemblies 70 are connected together sequentially by coupler pins 73 inserted into associated track bushings 72. A track shoe 75 is fixedly fastened to an end surface corresponding to the ground contact side of each link assembly 70 by a bolt and nut. More specifically, the bolt is passed through a bolt insertion hole 74 extending in a direction orthogonal to the axial direction of the coupler pin 73 at an intermediate position of the link piece 71. In the track 60, the track bushing 72 situated at a connecting portion of adjoining track links (link assemblies 70) engages with the teeth of a sprocket (not shown), and power supplied from the sprocket is transmitted, via the track bushing 72, to the coupler pin 73, to the pair of link pieces 71, 71, and then to the track shoe 75.
In the track 60 of the above-described type, the track bushing 72 that intermeshes with the teeth of the sprocket is fixed to the link pieces 71, 71. The track bushing 72 functions not only as a bearing member constituting a turning pair in the link assembly 70 but also as a power transferring member capable of direct transmission of power from the sprocket. Therefore, the external peripheral surface of the track bushing 72 receives a surface pressure caused by slide contact with the tooth surface of the sprocket and undergoes relative slipping occurring when engaged with the sprocket. However, since both ends of the track bushing 72 are fixed to the pair of link pieces 71, 71, the external peripheral surface of the track bushing 72 locally receives a surface pressure and relative slipping. Accordingly, in the conventional track 60, the track bushing 72 wears off at an early stage, and the frequency of replacement of the track bushing 72 is high, thereby producing the problem that running costs increase.
There are techniques known in the art (JP, 06-504747, B(1994) (published Japanese translations of PCT international publication for patent applications) and Japanese Utility Model Kokoku Publication No. 54-4206 (1979)), which are capable of providing solutions to the aforesaid problems. In such techniques, a track bushing is divided into three sub-bushings. Of these sub-bushings, one situated in the middle and brought into engagement with the teeth of a sprocket is formed so as to be rotatable (this sub-bushing is hereinafter referred to as a “rotatable bushing”). This construction allows the track bushing to engage smoothly with the teeth of the sprocket and reduces relative slipping at the time of engagement, thereby preventing the bushing from wearing off at an early stage. In addition to these prior art techniques, there is another technique that is disclosed in the Applicant's previous patent application (Japanese Patent Application No. 2003-220983).
In the track link according to the aforesaid 06-504747 technique, the track bushing is divided into three sub-bushings. Of these three sub-bushings, the intermediate sub-bushing is made rotatable on a coupler pin. As a result of such arrangement, the other sub-bushings situated on both sides of the intermediate sub-bushing are press-fitted into the inside of mount holes provided in the link pieces and serve as bearing bushings for the coupler pin. In this construction assembled as a link assembly, the pair of link pieces and the coupler pin are merely rigidly joined to each other, so that there occurs a high stress between the pin mount part and the track shoe mount part. Therefore, the track link of this technique presents the problem that rigidity falls considerably, in comparison with conventional tracks without rotatable bushings.
Additionally, in the link assembly connection part, for the provision of a seal member for providing sealing between a bushing end and a link piece located on the coupler pin fixing side, such a structure that a sealing member is fitted into the coupler pin side of the link is employed. This not only makes the machining work troublesome but also produces the problem that the assembling work becomes complicated. Furthermore, the aforesaid 54-4206 technique lacks strength on the coupler pin side of the link piece, which causes the same problem as does the 06-504747 technique.
The present invention was made with a view to providing solutions to the problems presented by the prior art techniques. Accordingly, a general object of the present invention is to provide a track with a rotatable bushing having strength rationally increased by link functionality sharing and by combination of such assigned functional tasks, and capable of achieving further improvements in the function of the rotatable bushing.